1. Field of Invention
The present invention relates to an energy storage device that can enhance the performances of batteries in numerous applications. More specifically, the present invention relates to the preparation of supercapacitors using iron oxide as the active material of electrodes.
2. Description of Related Art
Supercapacitor is also known as ultracapacitor or electric double layer capacitor. In rigid terms, although there is some distinction among them, they all can store a large quantity of charges up to several thousands farad (F) in compact size. Typically, when a capacitor with capacitance greater than 0.1 F, it is called a supercapacitor. Supercapacitor stores energy through surface adsorption of charges, which may or may not involve redox (reduction-oxidation) reactions. In either case, the adsorption only occurs at the electrode-electrolyte interface. As a consequence, supercapacitor can be charged and discharged very rapidly. With the capability of delivering high peak-current, supercapacitor can be a load-leveling device for batteries. On the other hand, supercapacitor may be the sole power source for loads requiring small currents.
The heart of supercapacitors is the electrode that determines the performances and cost of supercapacitors. Two components, an electronic conductor and an active material, form the electrodes. The former is to conduct electrons, and the latter is to adsorb charged species from the electrolyte. While the nature of the two components is important, the technique of disposing the said active material on the said conductor is also critical. Besides electrical requirements, the ideal electrodes should be constructed in monolithic structure without allowing the current collector to be in contact with the electrolyte during the lifetime of capacitors. There are three commonly employed methods in the literature: roller coating (cf. U.S. Pat. Nos. 4,737,889; 5,351,164; 5,558,680; 5,674,642), die molding (cf. U.S. Pat. Nos. 3,536,963; 4,717,595; 5,079,674; 5,136,473; 6,064,561) and dip coating (cf. U.S. Pat. Nos. 5,369,547; 5,464,453; 5,600,535; J. Electrochem. Soc., Vol. 143, pp124-130, 1996).
A binder, generally an organic polymer, is used in the first two methods so that the powdery active materials can be conglomerated. However, the binder will increase the resistance of the electrodes and impair the effective surface area of the active materials. In order to attain a sufficient thickness of deposition, dip coating has to be conducted many times with heat treatments followed each dip. The process is time-consuming, and the dipping solution is subjected to heat that will affect the quality of deposited films. Incidentally, both die molding and dip coating may yield monolithic electrodes.
As discussed in greater detail below, the present invention provide a method for fabricating economical supercapacitors for power applications. The method of the present invention to form a supercapacitor includes using iron oxide as the active material of electrodes, where iron oxide with a chemical composition of FexOyHz, where 1.0xe2x89xa6xxe2x89xa63.0, 0.0xe2x89xa6yxe2x89xa64.0, and 0.0xe2x89xa6zxe2x89xa61.0. The iron oxide can be directly grown on iron, steel and iron-deposited substrates.
An object of the present invention is to grow the FexOyHz film in chemical reactions using KMnO4 and NaNO3 as oxidizing agents. FexOyHz may be FeO, Fe3O4, Fe2O3 or FeO(OH) in which Fe3O4, the magnetite, is the major species that the high energy-storage capacitance is accredited to.
Another object of the present invention is to provide a conformal growth of iron-oxide film directly on substrates in the shape of plane, tube, angled frame and wrinkled plate. The substrates are submerged in solution during chemical oxidation, thus the iron-oxide film is deposited uniformly on every corner of the substrates.
Still another object of the present invention is to offer the growth of iron-oxide film on non-iron metallic substrates, as well as on nonconductors such as plastics, glass and ceramics. For the non-iron metals and nonconductors, metal iron has deposited prior to chemical oxidation. The deposition of iron metal can be achieved by electrolytic plating, electroless plating, chemical conversion, and combination thereof. As long as the iron metal can be successfully deposited on the substrates, it can be oxidized to the magnetite film easily.
Still another object of the present invention is to fabricate economical supercapacitors in various forms. Such supercapacitors can be integrated in the enclosure housings for lap top, hand-held computers, cellulars, power tools and batteries.
In other applications, supercapacitors of the present invention can be integrated with the frames of motorcycles and wheelchairs, with the body of automobiles, and with the light posts that use solar cells to generate electricity. When the supercapacitors are included in the housings, they can provide energy storage to the portable devices so that the batteries will have longer use-time, or the use of battery can be reduced. Similar benefits can be resulted from including in the frames and body of electric vehicles (EVs). Using the economical and light supercapacitors at the expense of expensive and heavy batteries, EVs will soon become practical and well accepted. As the energy storage device for green energies such as sun, wind, tide and heat, supercapacitors can virtually store any current generated by the nature, such currents may be inadequate for charging batteries.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.